Processes of this type are known from H. Strathmann "Trennung von molekularen Mischungen mit Hilfe synthetischer Membranen" (Separation of molecular mixtures with the aid of synthetic membranes) Publishers Dr. Dietrich Steinkopf Verlag, Darmstadt (Germany) 1979, pages 108 to 120. These known processes, however, have hitherto only resulted in filter membranes of the kind which have only a thin filtration active surface layer with pores measuring from 0.1 nm to 1 nm and which are mounted on a porous sub-layer serving merely as supporting substrate. Processes of the type mentioned above which are known from U.S. Pat. No. 4,229,241, DE-OS No. 2 554 932 and DE-OS No. 2 642 979 also result in such asymmetric filter membranes which have a thin surface skin as filtration active layer and only have pores of the size mentioned above. All these asymmetric filter membranes produced by the known processes described above are suitable for the separation of molecular mixtures by ultrafiltration or haemofiltration. They are not suitable, however, for the separation of particles from filtrates by microfiltration. Membrane filters for microfiltration and a method of producing such filter membranes have already been proposed in the earlier U.S. patent application Ser. No. 499,379 filed May 31, 1983 corresponding to German application No. 32 20 376, according to which aromatic polyamide is dissolved in at least two of the three possible solvents, viz. dimethylacetamide, dimethylformamide and N-methylpyrrolidone, and a solution mixture is prepared from such solutions in different solvents. When such a solution mixture of aromatic polyamide with at least two different solvents has been poured out to form a layer or membrane, the aromatic polyamide is precipitated by the action of moist air and the solvent is then washed out. This process, however, is complicated for large scale technical production and not trouble-free. It also has the disadvantage that considerable quantities of solvent get into the moist air in the process of coagulation and therefore seriously pollute the environment as well as giving rise to considerable difficulties in the recovery of the solvent.